This invention relates to the improvement of the method and apparatus for manufacturing soft metal sheaths for electrical cable wherein a soft metal tape such as aluminum tape or copper tape formed to a cylindrical tubular shape is applied on the periphery of an insulated core for the electrical cable; the butted edges are welded, e.g., by inert gas arc welding; the outer diameter of thus obtained metal tube is reduced; and the insulating core and metal tube are adhered to each other.
Methods are known for forming metal tape in the manufacturing of soft metal sheaths for an electrical cable. In one method, the tape is gradually shaped to a cylindrical form by passing it through a plurality of rollers. In a second method, the tape is arranged in the longitudinal direction and the lateral end portions or ears are formed to a cylindrical shape by means of a metallic forming tool like a shape of a cut open circular tubular pipe. It has been generally found, however, that friction and burning are caused between the tape and the tool, the tension force increased and undesirable faults are generated on the surface of the tape when forming such a soft metal tape by use of a forming tool of steel, particularly in the case of aluminum.
Next, as methods for welding the thus formed seam, the methods of high frequency induction heating or inert gas arc welding are well known. And further, as a method for metal sheathing the periphery of the finished insulated core, the processing by forming a metal tape to a cylindrical tubular shape and continuously welding the butted edges in the longitudinal direction is also known.
However, it is required in the known methods, as mentioned above, to make the inner diameter of the metal tube larger than the outer diameter of the insulated core so as to be able to provide an aperture for thermal insulation between the insulated core and the seam welding portion of the metal tube in order to prevent the melting of the insulated core by heating during the welding operation. Therefore, a working stage is necessary to join the periphery of the insulating core and the inner face of the metal tube to each other after welding.
In the prior methods of manufacture, the metal tube 20 has been reduced by drawing it through a sinking die 16 as shown in FIG. 7. This has caused problems, such as retaining of the lubricating oil supplied onto the metal 20 from a supplying device 15 to lubricate the inner surface of the die 16, causing slipping relative to the traction of the capstan 13 being the extracting means, causing variation in the drawing velocity and encountering troubles on the welding in the previous working stage. It has also been considered to provide a cleaning means to remove such oil, but this would require a very expensive installation and greatly increased manufacturing costs.